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The present invention relates generally to active sensors for electronically sensing the presence of an object and in particular to such a sensor having improved noise immunity.
The presence or absence of an object may be detected by measuring the interaction of the object with an electromagnetic field generated in a sensing volume. The object, when in the sensing volume, introduces a new or changed impedance into the circuit generating the electromagnetic field through capacitive or inductive coupling. Sensors that provide the source of the electromagnetic field used for sensing will be termed xe2x80x9cactivexe2x80x9d sensors.
In a capacitive presence sensor, for example, an object may increase a capacitive coupling between an electrode of the generating circuit and environmental ground return paths. In an inductive presence sensor, the object may inductively couple to an antenna of the generating circuit to change the effective inductance of that antenna.
This change in impedance, caused by the introduction of an object within the sensing area, is manifest as an energy transfer from the generating circuit to the object, such energy transfer being detected by a sensing circuit, for example, as increased current flow. The amount of energy transfer may be compared against a threshold to produce a binary, switched output indicating the presence or absence of an object within the sensed area.
Such electromagnetic field presence sensors do not require direct physical or electrical (ohmic) contact with the object and thus can be easily sealed against water and dirt for use in hostile industrial environments.
A tradeoff exists between the degree of sensitivity of such presence sensors and thus their ability to be triggered by small or remote objects, (e.g. a hand separated from the sensor by a thick glove), and their susceptibility to noise. As the sensitivity of the sensor is increased (increasing the sensing volume or decreasing the size of the object sensed) by setting the threshold to detect smaller energy transfers, there is an increased chance that electrical noise from the environment or conducted through the power line provided to the sensing circuitry will cause false triggerings of the sensor.
Averaging circuitry may be added to the sensing circuitry so as to diminish the effect of noise relative to the longer term signal generated and measured by the presence sensor. Such averaging circuitry, however, also slows the response of the presence sensor to changes in the presence or absence of an object it is detecting, thus limiting the application of such switches in cases where fast response is required.
The present inventors have recognized that electrical noise not only tends to be limited in the time domain, that is, to occur in bursts of limited duration, but that it is also limited in the frequency domain to occur, during any given burst, in a relatively narrow set of frequencies. Accordingly, an improved presence sensor can be constructed by applying to the sensing volume, a broadband electromagnetic signal and separately analyzing frequency bands of that signal to independently ascertain whether an object is present. Conflicts in these determinations at different frequencies, such as may be caused by electrical noise, is resolved by means of a voting circuit which adopts the output indicated by a majority of the determinations.
Specifically, the invention provides a method of sensing the presence of an object in a sensing volume including the steps of generating an electromagnetic signal composed of a plurality of different frequencies and electromagnetically communicating the electromagnetic signal to a sensing volume. Energy transfers to the sensing volume at the plurality of frequencies are separately detected and the energy transfers at the plurality of frequencies are compared to detect the presence of an object in the sensing volume and to provide an output signal.
Thus it is one object of the invention to provide a broadband presence sensor that may better resist frequency limited electrical noise.
The energy transfer at each frequency may be compared against a threshold indicating an energy transfer associated with the presence of the object to produce a frequency linked presence signal at each of the frequencies. The number of frequency linked presence signals indicating the presence of an object may be compared to the number of frequency linked presence signals indicating the absence of the object to determine the output signal. The comparison of the output signals observe a simple majority.
Thus it is another object of the invention to provide a simple voting method for eliminating artifacts caused by electromagnetic interference such as may provide a high degree of noise immunity even when multiple frequencies of the electromagnetic signal are obscured by electromagnetic noise.
The electromagnetic signal may be communicated to the sensing volume by an electrode capacitively coupled to an object in the sensing volume or by an inductor inductively coupled to the object in the sensing volume.
Thus it is another object of the invention to provide a technique that may be used for different types of electromagnetic presence sensors.
Each of the frequency linked sensor signals may be separately weighted in the comparison process.
Thus it is another object of the invention to provide a sensing of an object that is tailored to the particular frequency dependent characteristics of the object.
The amount of energy transfer may be detected by measuring changes in current or voltage at the different frequencies of the electromagnetic signal through or across a known impedance.
Thus it is another object of the invention to provide for a simple mechanism of measurement of energy transfer.
The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.